The UN body that advises world leaders on climate change must investigate an apparent bias in its report that resulted in several exaggerations of the impact of global warming, according to its former chairman.

In an interview with The Times Robert Watson said that all the errors exposed so far in the report by the Intergovernmental Panel on Climate Change (IPCC) resulted in overstatements of the severity of the problem.

Professor Watson, currently chief scientific adviser to the Department for Environment, Food and Rural Affairs, said that if the errors had just been innocent mistakes, as has been claimed by the current chairman, Rajendra Pachauri, some would probably have understated the impact of climate change.

The errors have emerged in the past month after simple checking of the sources cited by the 2,500 scientists who produced the report.

The report falsely claimed that Himalayan glaciers would disappear by 2035 when evidence suggests that they will survive for another 300 years. It also claimed that global warming could cut rain-fed North African crop production by up to 50 per cent by 2020. A senior IPCC contributor has since admitted that there is no evidence to support this claim.

The Dutch Government has asked the IPCC to correct its claim that more than half the Netherlands is below sea level. The environment ministry said that only 26 per cent of the country was below sea level.

A farmer in the Maghreb in Morocco, where the IPCC claims that harvests could decline by half in the next decade Photo: ALAMY

One more alarming claim in the IPCC's 2007 report is disintegrating under closer examination

Ever more question marks have been raised in recent weeks over the reputations of the UN's Intergovernmental Panel on Climate Change (IPCC) and of its chairman, Dr Rajendra Pachauri. But the latest example to emerge is arguably the most bizarre and scandalous of all. It centres on a very specific scare story which was included in the IPCC's 2007 report, although it was completely at odds with the scientific evidence – including that produced by the British expert in charge of the relevant section of the report. Even more tellingly, however, this particular claim has repeatedly been championed by Dr Pachauri himself.

Only last week Dr Pachauri was specifically denying that the appearance of this claim in two IPCC reports, including one of which he was the editor, was an error. Yet it has now come to light that the IPCC, ignoring the evidence of its own experts, deliberately published the claim for propaganda purposes.

A LEADING British government scientist has warned the United Nations’ climate panel to tackle its blunders or lose all credibility.

Robert Watson, chief scientist at Defra, the environment ministry, who chaired the Intergovernmental Panel on Climate Change (IPCC) from 1997 to 2002, was speaking after more potential inaccuracies emerged in the IPCC’s 2007 benchmark report on global warming.

The most important is a claim that global warming could cut rain-fed north African crop production by up to 50% by 2020, a remarkably short time for such a dramatic change. The claim has been quoted in speeches by Rajendra Pachauri, the IPCC chairman, and by Ban Ki-moon, the UN secretary-general.

This weekend Professor Chris Field, the new lead author of the IPCC’s climate impacts team, told The Sunday Times that he could find nothing in the report to support the claim. The revelation follows the IPCC’s retraction of a claim that the Himalayan glaciers might all melt by 2035.

Following an investigation by this blog (and with the story also told in The Sunday Times), another major "mistake" in the IPCC's benchmark Fourth Assessment Report has emerged.

Similar in effect to the erroneous "2035" claim – the year the IPCC claimed that Himalayan glaciers were going to melt – in this instance we find that the IPCC has wrongly claimed that in some African countries, yields from rain-fed agriculture could be reduced by up to 50 percent by 2020.

At best, this is a wild exaggeration, unsupported by any scientific research, referenced only to a report produced by a Canadian advocacy group, written by an obscure Moroccan academic who specialises in carbon trading, citing references which do not support his claims.

Unlike the glacier claim, which was confined to a section of the technical Working Group II report, this "50 percent by 2020" claim forms part of the key Synthesis Report, the production of which was the personal responsibility of the chair of the IPCC, Dr R K Pachauri. It has been repeated by him in many public fora. He, therefore, bears a personal responsibility for the error.